1. Field of the Invention
This invention relates to a polyolefinic composition containing a high content of inorganic filler, which is characterized by high impact strength, and relatively low heat of combustion, which allows ready burning of the composition. The composition of this invention is useful for such applications as corrugated cardboard, synthetic paper and synthetic wood.
2. Description of the Prior Art
Heretofore, the polyolefins, particularly polyethylene and polypropylene, have found a variety of uses as packaging materials, and as building and interior materials. Because polyolefins have excellent characteristics, processability, and are low in cost, their use in many areas has rapidly increased. However, the increased use of polyolefinic products has increased the pollution problem associated with the disposal of polyolefinic wastes. When polyolefinic materials are burned, they do not burn completely into gaseous products as do the cellulosic materials. Instead, they melt, forming an ill-defined mass of dense plastic. In addition, the high heat of combustion of polyolefinic materials requires a high furnace temperature which frequently results in rupture of the furnace walls. Furthermore, a large amount of soot is produced in the combustion process. Because of the difficulties associated with the combustion of polyolefinic materials, disposal of these materials is troublesome. Thus, the production of polyolefinic products has been limited because of the problems associated with their disposal.
In order to overcome these difficulties, the addition of an incombustible inorganic filler to the polyolefinic matrix has been proposed. However, because the affinity of the inorganic filler to the polyolefinic matrix is relatively weak, a high content of filler will cause the polyolefinic material to become brittle. Moreover, a high content of inorganic filler will not be as readily miscible in the polyolefinic matrix, and hence tends to decrease elongation and impact strength characteristics. Consequently, it is difficult to obtain an adequate polyolefinic composition having a filler content of greater than 50 weight percent.
It is well known in the art that the presence of a filler in a polyolefinic matrix results in increased bending stiffness of polyolefinic products. However, because of the loss in tensile elongation and impact strength, these compositions are unsuitable for use as corrugated cardboard and synthetic wood. Since it is theorized that these deficiencies are caused by the poor affinity of the filler to the polyolefinic matrix, it has been proposed to attempt to increase the affinity of fillers for polyolefinic materials by surface treating the filler to provide an oleophilic surface. However, it has been found that when conventional polyolefins, such as polypropylene and polyethylene are used, it is difficult to increase the affinity of fillers for the polyolefinic materials, even though the surface of the fillers have been treated with an oleophilic material. Accordingly, polyolefin-inorganic filler compositions having excellent bending stiffness, tensile elongation, impact strength and good balanced characteristics, have heretofore not been obtainable.
In U.S. Pat. No. 3,629,368, the assignee had disclosed a novel modified propylene polymer which is characterized by improved impact resistance and toughness at low temperatures as well as having a high softening point and stiffness. In one embodiment, that polymer was produced by polymerizing a propylene and subsequently continuously polymerizing ethylene after purging the polymerization system of propylene down to a pressure of less than 1.5 atmospheres. The polymerization is carried out at a temperature of 30.degree.-100.degree. C. and a pressure of 1-10 atmospheres utilizing a dialkylaluminum monohalide and titanium trichloride catalyst system. The ethylene is thus polymerized in the presence of a homopolymer of propylene, and 5 to 25% by volume of propylene in the vapor phase. The amounts of the ethylene and propylene fed to the reaction system are adjusted so that the resulting modified propylene polymer will contain from 5 to 40% by weight of ethylene, the overall polymerization reaction being conducted such that at least 300 parts by weight of polymer is formed per one part by weight of the titanium tetrachloride. The resulting modified propylene polymer is reported to be
a. a crystalline polypropylene PA1 b. a block copolymer wherein a propylene-ethylene random copolymer is end terminated onto the crystalline polypropylene PA1 c. a crystalline ethylene-propylene random copolymer having an ethylene content of more than about 90% by weight. PA1 a. a crystalline polypropylene PA1 b. a block copolymer wherein a propylene-ethylene random copolymer is chain-terminated onto said crystalline polypropylene PA1 c. a crystalline ethylene-propylene random copolymer having an ethylene content of more than about 90% by weight,
However, it was not recognized that the incorporation of a filler into this polymer would result in any effect which is different than the effects obtained by incorporating a filler into more conventional polyolefin compositions.